Embryology, Tongue

Development

The tongue begins to develop around the fourth week of intrauterine life. The first, second, third, and fourth pharyngeal arches contribute to the development of the various portions of the tongue. The development begins with the growth of a medial swelling from the first pharyngeal arch, known as tuberculum impar. Gradually, two lateral lingual swellings start to grow in the 5th week from the same arch. As the lateral swellings increase in size, they eventually merge and overlap tuberculum impar. This merging leads to the formation of the anterior two-thirds of the tongue. Since the mucosa overlying this area of the tongue has its origin from the first pharyngeal arch; it receives its sensory innervation from the mandibular branch of the V cranial nerve (trigeminal nerve).

Meanwhile, from the mesoderm of the second, third, and fourth pharyngeal arches, another median swelling, known as hypobranchial eminence, begins to develop and form the posterior third of the tongue. The mucosa overlying this area of the tongue receives its sensory innervation from the IX cranial nerve (glossopharyngeal nerve). The posterior-most part of the tongue develops from a third median swelling, arising from the fourth pharyngeal arch. This area of the tongue receives its innervations from the superior laryngeal nerve.

The muscles of the tongue predominantly derive from the myoblasts which originate in the occipital somites. They receive their innervations from the XII cranial nerve (hypoglossal nerve) except the palatoglossus muscle. The muscles of the tongue include extrinsic and intrinsic muscles. The extrinsic muscles are four in number (genioglossus, palatoglossus, styloglossus, and hyoglossus) and originate from the structures adjacent to the tongue. They allow the tongue to move in all directions. On the other hand, the four paired intrinsic muscles which include superior longitudinal, inferior longitudinal, verticalis, and transverses muscle have their origin as well as insertion inside the tongue. They are responsible for changing the shape of the tongue.

The first sign of development of taste bud on the lingual epithelium occurs at the 8th week of gestation. Between the ninth and eleventh week of gestation, many taste bud primordia develop. They differentiate into different cell types around the eleventh through the thirteenth postovulatory week. During this period, taste pores also develop.[1]

Cellular

The cells which form the tongue are hybrid in nature. The connective tissue component, as well as vasculature of the tongue, is derived from cranial neural crest cells (CNCC). These cells initiate the formation of the tongue bud and the interstitial connective tissue. The myoblasts which are responsible for the formation of the muscle components of the tongue derive from the occipital somite. Cells from this somite migrate into the primordium of the tongue, thus, forming the muscle cells in the tongue.[2]

Biochemical

Development of tongue is due to the complex interaction between various genes like Pax3, Pax7, and Dlx gene, which are responsible for survival and expansion of mammalian muscle and patterning information in tongue myogenesis. Some studies have shown that signaling by TGF Beta controls explicitly the proliferation of myogenic cells during tongue morphogenesis.[3]

Molecular Level

Development of tongue at the molecular level is influenced by molecular interaction between CNCC molecules and myogenic regulatory factors like myogenic factor 5 (Myf5), muscle-specific regulatory factor 4 (MRF4), myoblast determination protein (MyoD) and myogenin.[2] 

Function

The tongue participates in a variety of functions such as taste, speech, and food manipulation and cleaning of the oral cavity.

Taste Functions

The dorsal surface of the tongue is covered by a stratified squamous epithelium, with numerous papillae such as circumvallate papilla, fungiform papilla, filiform papilla, and foliate papilla. Taste buds which are intraepithelial chemosensory organs present within these papillae are responsible for taste perception. The circumvallate papilla carries the maximum number of taste buds. These taste buds via gustatory cell receptors interact with the chemicals present in the food and induce different taste sensations (sweet, salty, sour, and bitter).

Speech Functions

Various speech sounds require the interaction of the tongue with the teeth and different parts of the palate. The linguodental sounds such as “Th” require interaction between the tip of the tongue with the incisal surface of upper and lower incisors. The linguopalatal sounds may include the contact of the tongue with the anterior or the posterior part of the hard palate. When the tip of the tongue contacts with the anterior part of the hard palate, sounds such as “D, T, N, and, Z” is produced. When the tongue forms a valve and contacts the posterior part of the hard palate, it produces sounds like “ch" and "sh.” The velar sounds include a contact of the posterior part of the tongue with the soft palate. these sounds include “k" and "g .”

Food Manipulation Functions

The tongue aids in moving the food onto the occlusal surface of the teeth, mixing it with saliva as the food move away from the teeth, and in placing the food again on the teeth.[4] Thus, it helps in the formation of food bolus during the oral phase of deglutition. It also helps in propelling the food bolus beyond the anterior tonsillar pillar, which triggers the swallowing reflex.

Mechanism

Tongue musculature: The various functions of the tongue require the effective functioning of the extrinsic and intrinsic tongue musculature. The inferior fibers of the genioglossus muscle help in protrusion of the tongue, the middle fibers help in depressing the tongue, whereas the superior fibers of the muscle draw the tip of the tongue backward and downward.  The hyoglossus muscle depresses and retracts the tongue, whereas the styloglossus muscle elevates and retracts the tongue. The activation of palatoglossus muscle helps in elevating the posterior aspect of the tongue.

Among the intrinsic muscles of the tongue, the transverse muscle lengthens and protrudes the tongue whereas the vertical muscle flattens and widens the tongue. The superior longitudinal muscle runs in the longitudinal direction from their point of origin to insertion and aid to elevate the tip and lateral surface of the tongue, thus, shaping the dorsal surface of the tongue into a concavity. The inferior longitudinal muscle depresses the tip and lateral surface of the tongue, thus, shaping the dorsal surface of the tongue into convexity.

Taste buds: The taste buds present on the tongue contain taste villi. The stimulating substances present in the food or liquid interact with the taste villi.  The taste chemical binds to a protein receptor molecule that lies on the external surface of the taste receptor cell. This interaction opens ion channels allowing positively charged sodium or hydrogen ions to enter the cell leading to depolarization. The type of receptor protein determines the type of taste that will be perceived in each taste villus.

Testing

The strength and mobility of the tongue during protrusion is testable by using a tongue depressor. The tongue depressor is held vertically in front of the patients’ lips, and he/she is then asked to push the tongue against the depressor. Lateral movements of the tongue are also assessable by having the patient push the tongue against the depressor positioned to the right and left of the lips. The rating can be a mild, moderate, or severe weakness.[5] 

Pathophysiology

Developmental and structural abnormalities of the tongue are common features. The various morphological variations that may occur during the development of tongue are[5]:

• Aglossia
• Microglossia
• Macroglossia
• Ankyloglossia
• Cleft tongue
• Pentafid tongue
• Fissured tongue
• Geographic tongue
• Hairy tongue
• Median rhomboid glossitis

Aglossia: Congenital absence of the tongue is extremely rare. Usually, the tongue is absent in cases of gross underdevelopment or maldevelopment of the first visceral arches.

Microglossia: It is an uncommon developmental condition and is also known as hypoglossia. Its defining feature is a rudimentary or an abnormally small tongue. It leads to limited muscular movement and is associated with syndromes such as Hanhart syndrome.

Macroglossia: Macroglossia is an infrequently encountered condition characterized by tongue enlargement, seen in association with other congenital defects leading to syndromes such as down syndrome (trisomy 21), Beckwith-Wiedemann syndrome.

Ankyloglossia It occurs due to failure in cellular degeneration leading to a longer anchorage between the tongue and floor of the mouth; this is commonly known as "tongue-tied" and demonstrates an abnormally short lingual frenulum. Ankyloglossia can range in severity from mild to complete ankyloglossia in which the tongue gets fused to the floor of the mouth restricting its free movement.[6] A short lingual frenulum leading to tongue-tie is also associated with several genetic syndromes such as related Robinow syndrome, oral-facial-digital syndrome Type I, Opitz syndrome, and Van der Woude syndrome.

Cleft tongue: It is also known as bifid tongue and occurs when the lateral swellings fail to merge. It can be partial or complete. The former is a more common entity and is manifested as a deep groove on the dorsal surface of the tongue in the midline. It occurs when the mesenchymal proliferation interferes with the merging leading to failure of the obliteration of the groove. There are reports of bifid tongue in syndromic cases like Opitz G BBB syndrome, oral-facial-digital syndrome type I, Klippel–Feil anomaly, and Larsen syndrome.[7]

Pentafid tongue: Disturbance in the mesodermal penetration and mesenchymal fusion during the development of tongue development is responsible for this malformation.

Fissured tongue: It is also known as scrotal tongue or lingua fissurata. It is congenital anomaly manifested as grooves oriented anteroposteriorly on the dorsal aspect of the tongue with multiple branches extending towards the lateral aspect. The grooves range from 2 to 6 mm in depth. In a severe form of the fissured tongue, when the grooves are extremely prominent and interconnected, the tongue may appear to be lobulated. It can also present in association with down syndrome or Melkerson-Rosenthal syndrome (a triad of fissure, granulomatous cheilitis, and cranial nerve VII paralysis).

Geographic tongue: Geographic tongue, also known as lingua geographica or benign migratory glossitis is an inflammatory disorder caused by loss of filiform papillae.

Hairy tongue: It is also known as a black hairy tongue and characteristically demonstrates the accumulation of excess keratin on the filiform papillae on the dorsal surface of the tongue leading to the formation of elongated strands resembling hair. This condition most commonly affects the midline just anterior to the circumvallate papillae, usually sparing the lateral and anterior borders.

Median rhomboid glossitis: It is a condition that presents in the midline of the dorsal surface of the tongue, just in front of the circumvallate papillae. It presents as a well-demarcated, symmetric, depapillated area. However, it also occasionally appears in the paramedial location.

Clinical Significance

In infants, macroglossia is manifested by noisy breathing, drooling, and difficulty while eating. It can also result in a lisping speech. The pressure exerted by the large tongue against the mandible and teeth can produce mandibular prognathism and open bite. Cleft tongue if present can lead to difficulty while eating.  In breastfeeding infants, ankyloglossia can cause feeding problems resulting in untimely weaning. It may also lead to speech defects and dental problems.  It leads to the blanching of soft tissue during extrusion of the tongue and also exerts force on mandibular anteriorly. Moreover, it interferes in the process of tooth brushing, favoring the risk of plaque accumulation, followed by tissue inflammation and gingival recession.